1. Field of the Invention
The present invention relates to an apparatus for wirelessly transmitting and receiving multicast data. More particularly, the present invention relates to an apparatus for efficiently and wirelessly transmitting and receiving multicast data having a scalable data structure, methods thereof, and a wireless communication system including the transmitting apparatus and the receiving apparatus.
2. Description of the Related Art
Multicast means sending the same information to a plurality of receivers (hereinafter, referred to as receiving apparatuses) with a single transmission. Multicast technology is advantageous in efficiently using network resources without wastage and is thus used for ultra high-speed data.
In transmitting data (hereinafter, referred to as multicast data) through multicast, the amounts of data that each one of the receiving apparatuses can receive through a network per unit time, that is, the data rates of the receiving apparatuses, may be different. Here, when a transmitting apparatus transmits data at a fixed data rate, the transmitting apparatus must transmit data at the lowest data rate among the data rates of the receiving apparatuses so that all of the receiving apparatuses are able to receive the data. If the transmission data rate is increased, receiving apparatuses having a lower data rate than the transmission data rate cannot receive data transmitted from the transmitting apparatus.
In order to overcome this problem, there has been proposed a method of dividing multicast data into several streams, transmitting only some of the streams to receiving apparatuses having a lower data rate than a transmission data rate, and transmitting all of the streams to receiving apparatuses having a higher data rate than the transmission data rate. In order to effectively perform this method, a part of the data (i.e., basic data, like video or audio data) needs to include basic information, which can provide original data of low quality, and the remaining part of the data (i.e., additional data) may include more detailed information to be added to the basic data so that original data of high quality can be provided. Such a data structure is defined as being scalable. Motion Pictures Experts Group 4 (MPEG4) is a representative coding method using such scalability.
Unbalanced modulation is a scheme for transmitting multicast data having a scalable data structure in a wireless transmitting system. Representative unbalanced modulation schemes include Unbalanced Quadrature Phase Shift Keying (UQPSK) and Unbalanced Quadrature Amplitude Modulation (UQAM). These schemes are the same when a single signal element has two bits, and thus only UQPSK will be described below with reference to the attached drawings.
FIG. 1A illustrates a block diagram of a communication system using UQPSK. Referring to FIG. 1A, a communication system using UQPSK includes a transmitting apparatus 100 and a receiving apparatus 110. For clarity of the description, a communication channel 120 is illustrated in FIG. 1A.
The transmitting apparatus 100 modulates a basic data signal 1 and an additional data signal 2 using a sine wave 3, D1×sin(ωct), and a cosine wave 4, D2×cos(ωct), having the same frequency to generate a basic modulated signal 5 and an additional modulated signal 6 and adds the basic modulated signal 5 and the additional modulated signal 6 to generate a transmission signal 7.
The transmission signal 7 is transmitted to the receiving apparatus 110 through the communication channel 120. During the transmission, noise is added to the transmission signal 7, which results in a noisy transmission signal 17.
The receiving apparatus 110 demodulates the noisy transmission signal 17 using a sine wave 13, sin(ωct), which is synchronized with the sine wave 3 of the transmitting apparatus 100, and a cosine wave 14, cos(ωct), which is synchronized with the cosine wave 4 of the transmitting apparatus 100, to generate a basic data signal 11 with noise and an additional data signal 12 with noise.
FIG. 1B illustrates a constellation I/Q plot with respect to a transmission signal in the UQPSK modulation scheme. Referring to FIG. 1B, the basic modulated signal 5 and the additional modulated signal 6 are the in-phase (I) component and the quadrature (Q) component, respectively, of the transmission signal 7. Here, the magnitude of the I component is the same as the amplitude D1 of the sine wave 3, and the magnitude of the Q component is the same as the amplitude D2 of the cosine wave 4. When D2=λ×D1, λ has a value between 0 and 1 in the UQPSK modulation scheme. When λ is 0, binary PSK (BPSK) is performed. When λ is 1, unbalanced QPSK is performed.
Referring to FIG. 1B, when D1 and D2 are fixed in the transmitting apparatus 100, the receiving apparatus 110 can selectively receive a Q component according to the magnitude of noise added to a received signal. In other words, when a signal-to-noise ratio (SNR) is high and the magnitude of noise is less than the amplitude D2 of a Q component, the receiving apparatus 110 receives both I and Q components and combines them to obtain original data in which additional information is added to basic information. However, when an SNR is low and the magnitude of noise is greater than the amplitude D2 of a Q component, the receiving apparatus 110 receives only an I component and obtains only basic data.
Referring to FIG. 1B, upon transmission, data in the transmission signal 7 is identifiable as one of four representative signal points 21, 22, 23, and 24 in the I/Q constellation. However, after propagation through the communications channel 120, the receiving apparatus 110 receives a noisy transmission signal 17 which can change the locus for the signal points in the I/Q constellation. For example, if the receiving apparatus 110 receives a signal that corresponds to the signal point 25, it is necessary to determine whether the original transmission signal 7 corresponds to the signal point 21 or 22 in order to receive the Q component. For a case where the original transmission signal 7 actually corresponds to the signal point 22 having a magnitude of noise added to the Q component of n2, if the receiving apparatus 110 determines that the transmission signal 7 corresponds to the signal point 21 and that the magnitude of noise added to the Q component is n1, an error occurs. Accordingly, as the amplitude D2 of the Q component increases, the probability of an error occurring due to noise decreases, and thus more receiving apparatuses can receive additional data.
However, since transmission power must be constant in the UQPSK modulation scheme, the I component must be decreased when the Q component is increased. Referring to FIG. 1B, signal points must be located on a unit circle 26 to maintain a constant transmitting power. In such a plot, when D2 is increased, D1 is decreased. Accordingly, when the magnitude of the Q component is increased for a group of first receiving apparatuses having a high SNR, a group of second receiving apparatuses having a low SNR may not receive the I component because the magnitude of noise is greater than the amplitude D1. Such a state in which a receiving apparatus having a low SNR cannot receive basic data transmitted from a transmitting apparatus is referred to as an outage.
In other words, in a method of transmitting multicast data having a scalable data structure using an unbalanced modulation scheme, basic data and additional data are transmitted using a single method, and a receiving apparatus determines whether to receive the additional data according to a physical state thereof. Accordingly, when all receiving apparatuses increase a data rate to increase the amount of received data, an outage occurs and thus a data rate cannot be increased any further.